The present invention relates to a method and a device for resistance welding by a spot welding gun, for instance for plate works, such as repairs of car bodies or the like.
Resistance welding or spot welding is a method by which two or more welding objects are joined together at one or more points.
Resistance welding of this kind may be used for manufacture but more generally it is used for repairs, particularly of car bodies. Resistance welding is in this respect often the most suitable joining method, where it is applicable as regards accessibility. It is a fast and effective method, which provides for stronger welding joints with fewer welding spots than if a MIG-weld or the like is used. Resistance welding using a spot welding gun is furthermore harmless to the environment; for instance the flue gases are severely reduced. Furthermore, the method provides for a low heat release in the plate around the welding spot, which provides for a retained corrosion resistance of zincified plate, and the thermal stresses in the structures are minimized.
The parameters that may be varied during the welding include electrode pressure, welding current strength and time, cooling effect, cooling time and hold time.
There are two different kinds of spot welding guns, X-guns and C-guns. The X-gun has a pneumatic cylinder, which via a lever affects the electrode. The electrode pressure is thus dependent on the lever length, which in turn affects the rest of the welding parameters.
The spot gun, to which the present invention relates, is of the C-gun type, in which the pneumatic cylinder affects the electrode directly. The electrode pressure is constant, independent of the form of the arm.
One of the problems of a conventional C-gun is that, when positioning the welding gun at the welding objects, it is difficult to pass edges or other obstacles of the welding objects due to the relatively short stroke of the cylinder.
Typically, this is solved in such a way that it will be possible for the arm to be opened or demounted. Thus, it is known to pivot a spot welding gun, called Prospot PR-10, laterally, while the arm and the front electrode are kept fixed, and thus create a larger space between the electrodes, whereby possible obstacles of the welding objects may be passed in order to then pivot the gun back to the original position. See for instance the information that was publicly available on Sep. 9, 1998 on the Internet (wwe.prospot.com/uarms.htm).
It is also known to have the engagement of the arm in the gun radially displaced from the electrodes and turn/rotate the arm with the front electrode, whereby the space between the electrodes is created. See for instance the C-gun with article number 8652 of LORS-TECNA. The arm is also demountable. In order to turn or demount the arm four screws must, however, be released. Furthermore, in all the instances described above, the electrodes must be centered afterwards.
Another problem is that the electrode distance sometimes has to be changed as a consequence of the fact that the appearance, i.e. the thickness, of the welding objects has changed. This is, according to the known arm, solved by the front electrode being exchangeable and/or movable.
Thus, particularly if both obstacles have to be passed and the electrode distance has to be adjusted, extensive working operations are needed, which will severely reduce the capacity of the spot welding gun.
It is an object of the present invention to provide a spot welding gun lacking one or more of the problems which may appear using a spot welding gun according to the known art.
Thus, it is an object to provide a spot welding gun, which through an uncomplicated process enables or simplifies the passage of obstacles during positioning of the spot gun at the welding objects and during removal of the gun therefrom, while maintaining the electrode centering.
A further object of the invention is to provide a spot welding gun, which moreover allows adjustment of the electrode distance, while the number of working operations is reduced to a minimum.
Yet a further object of the invention is to provide a spot welding gun of the kind mentioned above, which is compact and reliable.
These and other objects are realized according to one aspect of the invention by a spot welding gun for resistance welding of objects comprising a C-shaped arm, a front and a back electrode and a cylinder, which is driven pneumatically or the like, the back electrode being axially movable a limited distance by the cylinder. The spot welding gun is arranged to clamp the objects between the electrode tips by impelling the cylinder forwards and to flow a welding current between said electrode tips. According to the invention the back electrode is arranged to be releasable from, axially movable in relation to, and lockable to the cylinder.
The inventive method comprises, before the welding, releasing the back electrode from the cylinder, moving the back electrode axially backwards enough to enable or simplify the positioning of the spot welding gun, positioning the spot welding gun so that the electrodes are on each side of the objects, moving the back electrode axially forwards to a position where its electrode tip is at a distance from the front electrode less than the sum of the stroke of the cylinder, and the thickness of the objects, and fastening the back electrode to said cylinder.
The method enables or simplifies the passage of obstacles during positioning of the spot welding gun and the method for adjustment of the electrode distance in one and the same operation, which will very considerably simplify the handling of the spot welding gun. Furthermore, the electrode centering will be maintained during the entire working operation.
The method after welding comprises releasing the back electrode from the cylinder, and moving the back electrode axially backwards enough to enable or simplify the removal of the spot welding gun from the welded objects.
The back electrode is preferably able to be moved axially backwards at least to a position where its electrode tip is inside the cylinder, and forwards at least to a position where its electrode tip is at a distance from the front electrode less than the sum of the stroke of the cylinder and the thickness of the thinnest object for which the gun is intended to be used.
Preferably, the back electrode is arranged to be released manually and quickly from and fastened to the cylinder, via a locking lever, which will further simplify the working operation of the gun.
The C-formed arm with the front electrode may be radially turnable/rotatable around the cylinder, and/or demountable.
An advantage of the spot welding gun according to the present invention is its manageability, which provides for a high working capacity.
A further advantage of the invention is that the centering of the electrodes is maintained. No advanced centering mechanisms or fastening mechanisms are thus needed.
More advantages of the invention will be apparent in the description below.